So, just thinking out loud really: What are the results of a Celerium shortage? What are the second or third order consequences that pop up? To get started:
Witchcraft is probably going to SKYROCKET in popularity. We don’t have details on how wide exactly “opposed” is, like if an Air familiar would leave you weaker with just Earth based spells or all non-air, but having a weaker capacity is far better than taking the risk of Will-break from a too small Celerium conduit or an imperfect gemstone one. It’s not a case of preference for specialty anymore, it becomes a case of how badly you want to risk your life. This is going to have a lot of knock-on effects, like how magical services/items work. Possibly we’ll see more centralisation/collectives being formed to counterbalance the forced specialisation? A union of witches can share their specific knowledge much easier and have others cover their own weaknesses in all senses of the term.
Geopolitically, there is going to be upheval and that’s likely to be a later-book problem for Siobhan (because of course she’s going to get tangled in it, whether she likes it or not). High grade Celerium is the fulcrum, along with the modern magic that the Haze War produced. They’re shaping up for a WW1 style situation and it’s going to be ugly. Depending on how harshly it cuts, there could be shortages in an already low-production system, leading to knockon effects as magical infrastructure just can’t be maintained. They use Celerium in certain spell arrays too, after all, although that would likely be mostly shattered or low grade, whereas the problem here is mostly higher grade.
On the practical level for Siobhan, we’ve got the motive for her chasing down her mother’s ring, since that’ll solve the issue for her quite efficiently, although the Conduits she stole off the Pendragons will probably be a good stopgap. Also possibly explore the bloodline stuff, although that’s it’s own can of worms really. It does look like it’s viable though, especially given the hint that one of the atrocities committed against the people was forced familiar contracts on sapients. Which implies that they, or at least some of them, could channel magic in a similar way to magical beings.
I’m just going to make some guesses based on how oil (and kerosine) functioned in the industrial revolution. Seems likely that:
Witches would be easier to regulate.
Familiars aren’t as good for channeling magic.
Enchanting may require cerelium, even if you have a familiar.
Military Dictatorships will be very worried about resources and become aggressive (e.g. Japan trying to secure the Pacific so it could monopolize oil and rubber shipments) S. think magic is the most important resource of the military.
Magic appears to be used in food preservation, and is used in mining: both important to a healthy civilization.
Magic is essential to healthcare; sickness will be more difficult to treat and pandemics could become an issue
Magic is not a minor part of long distance communication networks - affecting commerce and the military
Class mobility is affected, and innovation may be further restricted
I mean, one other consequence is that power grows less centralized eventually. Thaumaturges are essentially the wealthy elite who can afford all of the celerium and components and study.
As the celerium supply decreases and a smaller population of active sorcerers can be maintained by that elite, the importance of other manufacturing methods etc. grows, and the relative power of the common people, who will run larger and more important sectors of society also grows.
Basically, Oliver’s goals are inevitable in the long term because nobody would be able to afford not to utilize their populations more efficiently without the extra magic to back it up.
Osham might already be actively pursuing shifting their infrastructure away from magic so it’s more resilient to shortages.
I think Kiernan more or less answered the question: for civilian purposes, diamond or sapphire would suffice, and they can be artificially made with thaumaturgy. But for military applications there’s really no good substitute for celerium, so it’s going to become a major military asset.
And yet, it doesn’t seem like the Crowns are doing anything to encourage lower ability thaumateges (like, say, Newton’s mom, or other “hedge-witch” strength) to switch to using diamond or other gems. While sure, someone at the level of Master can grab a random rock and make it a diamond, it doesn’t seem to be significantly cheaper than the flawed celerium S buys early on.
Now, part of this might be because the Crowns aren’t eager to let Osham and other countries realize the celerium mines are running low - that might kick off Haze War 2, after all. But it also seems like it would be a major societal shift as well - everyone thinks you need celerium to cast.
I wouldn’t be surprised if the Crowns don’t want more low-level thaumaturges to exist at all!
Back to diamonds though - creating diamonds is clearly not that difficult, since Lacer assigns it as a teaching exercise to Sebastien. And to make it even easier they could use graphite instead of air. Once the celerium shortage gets really bad, that’s probably what people will start doing. Even if you need a diamond the size of a basketball to cast Master-level spells…that doesn’t really affect civilian use all that much. Most sorcerers are using big elaborate Circles with fancy Word arrays anyway, so it’s not that much more of an inconvenience to have a bulky Conduit. The people who would be seriously affected would be freecasters, but they’re a very small and elite group; they’ll be fine.